Major reviewThe Eye as Metronome of the Body
Section snippets
Circadian Rhythms
Every day at approximately 6:00 a.m. our serum cortisol level peaks.35 How does our body control the spurt of cortisol release so that it occurs at the same time and amount every day? It is thought that our body has an internal clock with a free-running period of about 25 hours51, 98 and that this endogenous rhythm is entrained to the 24-hour (solar) period.24 In addition to cortisol,24, 35, 37, 51, 108 other hormones such as growth hormone,36 melatonin,33, 45 follicle-stimulating hormone, and
Retinohypothalamic Tract
The retinohypothalamic tract (RHT) is a direct afferent pathway from the retina to several hypothalamic nuclei. This tract sends visual signals, including those from the light/dark cycle, to the hypothalamus, which in turn uses this information to pace its clock and synchronize the endogenous circadian rhythm. The RHT has been described in humans80, 81, 82, 84 as well as several experimental animal models, such as the rat,69, 90, 94 guinea pig,69 golden hamster,10, 70, 75, 95 and other rodents.
Hypothalamus
When the SCN is destroyed in rats, circadian rhythms of hormonal release55 and the sleep-wake cycle disappear.28 Transplantation of fetal SCN into the same host restores rhythmical behavior according to the donor's activity.39, 83 The SCN is proposed to be one and perhaps the only pacemaker of the multioscillator system.59 After ablating the SCN in rats, Stephan and Zucker93 demonstrated a loss of circadian rhythm in the rats' drinking behavior and locomotor activity. In a similar study, Moore
Circadian Rhythm Abnormalities in Blind Persons
Environmental cues, such as social interactions, clocks, regular activity, and feeding, might provide the cues necessary to establish normal circadian rhythms in blind individuals. However, fixed-interval feeding in blind rats did not entrain the circadian pacemaker.20 Likewise, many blind persons without light perception exhibit free-running temperature,18 cortisol, and melatonin levels,45, 47 despite having a daily routine.35, 51 Circadian rhythm abnormalities occur and there is a difference
Conclusion
The importance of light entrainment of circadian rhythms is becoming more and more apparent as further information is unraveled regarding the role of SCN in other neuroendocrine functions, such as thyroid hormone, growth hormone, and GnRH. Controversies regarding the critical nature of light/dark cycle input to neuroendocrine systems such as fertility remain unresolved. Future studies comparing large clinical populations with different degrees of vision will be important.
Circadian rhythms are
Method of Literature Search
A Medline/Ovid search was conducted for all database years (1968–present). Search words were: circadian and vision, diurnal and vision, retino-hypothalamic, circadian rhythms, retinohypothalamic tract, diurnal rhythms, visual entrainment, hypothalamus. These searches were limited to English journals and abstracts. From this bibliography we proceeded with a classical search of primary sources. On two occasions we obtained help in translating from the German.
Acknowledgements
The authors have no commercial or proprietary interest in any product or concept discussed in this article. They wish to acknowledge Mona Khan, MD, for having persistently questioned and researched many of the issues of light entrainment.
References (113)
- et al.
Retinally perceived light can entrain the pineal melatonin rhythm in Japanese quail
Brain Res
(1991) - et al.
Melatonin stabilises sleep onset time in a blind man without entrainment of cortisol or temperature rhythms
Neurosci Lett
(1990) - et al.
Specific in vivo binding of [125I]-iodomelatonin to melatonin receptors in rat brain
Neuropharmacology
(1990) Circadian rhythmicity in cultured liver cells. I. Rhythms in tyrosine aminotransferase activity and inducibility and in (3H) leucine incorporation
Int J Biochem
(1973)- et al.
Loss of circadian rhythm in sleep-wakefulness cycle in the rat by suprachiasmatic nucleus lesions
Brain Res
(1975) - et al.
Pineal rhythms are synchronized to light-dark cycles in congenitally anophthalmic mutant rats
Brain Res
(1999) - et al.
Retinohypothalamic projections in the hamster and rat demonstrated using cholera toxin
Brain Res
(1988) - et al.
Vasopressin-containing neurons of the suprachiasmatic nuclei inhibit corticosterone release
Brain Res
(1992) - et al.
Identification of retinal ganglion cells projecting to the lateral hypothalamic area of the rat
Brain Res
(1997) Fertility of blind women
Fertil Steril
(1982)
Fertility and menopause in blind women
Fertil Steril
Pineal effect on longevity
J Chronic Dis
Loss of a circadian adrenal corticosterone rhythm following suprachiasmatic lesions in the rat
Brain Res
SCN output drives the autonomic nervous systemwith special reference to the autonomic function related to the regulation of glucose metabolism
Prog Brain Res
Differences of somatostatin mRNA in the rat suprachiasmatic nucleus under light-dark and constant dark conditionsan analysis by in situ hybridization
Neurosci Lett
Morphological characteristics of retinal ganglion cells projecting to the suprachiasmatic nucleusa horseradish peroxidase study
Brain Res
Anterograde tracing of retinal afferents to the tree shrew hypothalamus and raphe
Brain Res
A retinohypothalamic pathway in manlight mediation of circadian rhythms
Brain Res
Retinohypothalamic projectionelectrophysiological evidence for the existence in female rats
Brain Res
Transplantation of the neonatal suprachiasmatic nuclei into rats with complete bilateral suprachiasmatic lesions
Neurosci Res
A second hypothalamic nucleus receiving retinal input in manthe paraventricular nucleus
Brain Res
Influence of environmental light-dark cycle and enucleation on activity of suprachiasmatic neurons in slice preparations
Brain Res
Altered diurnal serum melatonin rhythm in blind men
Lancet
Melatonin receptorscurrent status, facts, and hypotheses
Life Sci
Direct vasoactive intestinal polypeptide-containing projection from the suprachiasmatic nucleus to spinal projecting hypothalamic paraventricular neurons
Brain Res
Rhythmic changes in rate of the heart and heart cells during prolonged isolation
Comp Biochem Physiol
Circadian control of reproduction in the female rat
Prog Brain Res
Circadian metabolic patterns in cultured hamster adrenal glands
Comp Biochem Physiol
Comparative physiologydiurnal rhythms
Ann Rev Physiol
Influence of blindness on plasma luteinizing hormone, follicle- stimulating hormone, prolactin, and testosterone levels in prepubertal boys
J Clin Endocrinol Metab
Diurnal rhythms of serum gonadotropins, testosterone, estradiol and cortisol in blind men
J Clin Endocrinol Metab
Human melatonin regulation is not mediated by the three cone photopic visual system
J Clin Endocrinol Metab
The effect of optic enucleation on the male albino rat
Anat Record
Suprachiasmatic nucleus lesion increases corticosterone secretion
Am J Physiol
Influence of subcutaneous deposits of melatonin on the antigonadotrophic effects of blinding and anosmia in male rats. A dose-response study
Neuroendocrinology
Retinohypothalamic projections in the common marmoset (Callithrix jacchus)a study using cholera toxin subunit B
J Comp Neurol
Periodic exposure to a brief light signal stimulates neuroendocrine-gonadal activity in golden hamsters
J Androl
Effects of constant darkness and constant light on circadian organization and reproductive responses in the ram
J Biol Rhythms
Sterility of blind women
Jpn J Fertil Steril
Diurnal cycles in plasma FSH, testosterone and cortisol in men
J Clin Endocrinol Metab
Review of Horners syndrome and a case report
Optom Vis Sci
Effect of light on sexual maturation, estrous cycles, and anterior pituitary of the rat
Endocrinology
Circadian rhythm of body temperature persists after suprachiasmatic lesions in the squirrel monkey
Am J Physiol
Fixed interval feeding does not entrain the circadian pacemaker in blind rats
Am J Physiol
Synaptology of the rat suprachiasmatic nucleus
Cell Tissue Res
Cortisol is secreted episodically by normal man
J Clin Endocrinol Metab
Pineal glandinfluence on gonads of male hamsters
Science
The influence of light via the eyes on animals and man
Ann NY Acad Sci
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The U-shaped association between self-reported sleep duration and visual impairment in Korean adults: a population-based study
2016, Sleep MedicineCitation Excerpt :The SCN acts as a biological master clock to synchronize several biological rhythms (eg, sleep–wake cycles) into a 24-hour day cycle [33]. Because humans have intrinsic circadian rhythms longer than 24 hours [34], the clock must be continually reset to maintain synchrony with the outside world [35]. If light input is reduced or becomes absent due to low vision or blindness, SCN becomes incapable of adjusting to the day–night cycle, and sleep–wake cycles become desynchronized [36]; this is known to cause recurrent insomnia and daytime sleepiness [37].
The optic chiasm
2016, Taylor and Hoyt's Pediatric Ophthalmology and Strabismus, Fifth EditionThe optic chiasm
2012, Pediatric Ophthalmology and Strabismus: Expert Consult - Online and PrintThe role of environmental light in sleep and health: Effects of ocular aging and cataract surgery
2010, Sleep Medicine ReviewsCitation Excerpt :In mammals the master circadian clock resides within the paired suprachiasmatic nuclei of the anterior hypothalamus.2 The SCN originate daily patterns of most physiologic and hormonal processes,13,14 timing events to allow preparation for anticipated metabolic and physical activities.15,16 Prior to habitual awakening the SCN initiate actions critical in transitioning from sleep to wakefulness including hepatic and adrenal stimulation which increases serum glucose and produces a morning cortisol surge.15,16
Blue-blocking IOLs Decrease Photoreception Without Providing Significant Photoprotection
2010, Survey of OphthalmologyCitation Excerpt :Their axons form the retinohypothalamic tract which conveys nonvisual information from the retina to the SCN and other nonvisual brain centers.42 Deficiencies of pRGC photoreception cannot be perceived consciously.66 The master circadian clock of mammals resides within the paired SCN of the anterior hypothalamus.42
Vision and the hypothalamus
2010, Optometry